The 1939 James Thurber story The Secret Life of Walter Mitty occupied just two pages of the New Yorker magazine. A miracle of comic understatement and concision, it does not necessarily prove Shakespeare's maxim that brevity is the soul of wit, though it's a powerful argument in its favour.

The new film version, directed by its star Stiller, is an equally powerful argument that long-windedness is the soul of witlessness. A riot of CGI tramples a thin storyline that, in essence, is like one of those personal training videos that bellow "you can do it" at people who probably can't.

Thurber created a henpecked, daydreaming husband, sent to buy rubber overshoes while his wife is having her hair done, who fantasises being respectively a fighter pilot, surgeon, crack shot and soldier hero. A 1947 adaptation, which Thurber himself scripted, made it a memorably amusing musical vehicle for the versatile genius of singer-comedian Danny Kaye.

The new version, inevitably, introduces a love interest (Wiig). She works with Mitty at Life magazine where he is the "negative assets manager" (read: runs the photo library) and gazes longingly at her. Among the many complications stuffed into the plot is that the magazine is about to run its last print issue and Mitty can't find the negative for the cover shot by legendary adventurer-photographer, Sean O'Connell (Penn).

Cue a worldwide search in which the mild-mannered librarian scales mountains and leaps from helicopters into shark-filled waters.

That this lost negative, the pretext for the entire story, turns out to be the lamest of shaggy dog jokes, is not the most disappointing feature of the film. The original idea has been diluted so that Mitty's daydreams are very brief, effects-heavy inserts in the story of a geeky man pining for a pretty colleague's love. In other words, this is a Mitty for people with short attention spans (which is pretty funny since the original was barely 2000 words); it's a Mitty for the age of the tweet and text message.

To say more is to pay undue attention to a deeply depressing example of Hollywood's skill at turning the sublime into the banal, but it is worth wondering who came up with the idea that the milquetoast Walter Mitty was once a teenage skateboard whizz with a mohawk - and what that person was smoking at the time ...